Here's why:

* Compound: A substance formed when two or more different elements are chemically bonded together in a fixed ratio.

* Element: A pure substance consisting of only one type of atom.

* Mixture: A combination of two or more substances that are not chemically bonded and can be separated by physical means.

Hydrogen chloride (HCl) is formed by the chemical bonding of hydrogen (H) and chlorine (Cl) atoms in a 1:1 ratio. This means it's a compound, not an element or a mixture.